Controls on the distribution of Mnium arizonicum along an elevation gradient in the Front Ranges of the Rocky Mountains, Alberta1

endemic to North America and in the northern Rocky Mountains it occurs most abundantly in the understory of subalpine tree islands formed by Abies lasiocarpa and Picea engelmannii. This species reproduces almost entirely through apical fragment regeneration and its expansion into lower elevation sites may be dispersal limited. Mnium arizonicum was experimentally dispersed along an elevation gradient as apical fragments and colony transplants. Individual performance was expected to interact with factors that vary with elevation such as habitat characteristics and competition intensity. Habitat characteristics were assessed at the site level by detailed site mapping and at the scale of the study area by restricted-random 5 m radius plots. The interaction between M. arizonicum and Hylocomium splendens was investigated for a switch from competition to facilitation along the elevation gradient. The experimental design included six elevation classes with three replicate sites and within each site two microsites (Picea tree base and Hylocomium carpet) each with three replicate plots. At sites where M. arizonicum was most abundant, it was excluded from the smallest tree islands with higher tree densities. In the region of the study area, the occurrence of M. arizonicum was most probable around tree bases located at 2000+ m with a thin (< 3 cm) litter layer and with one or more other bryophyte species present at the same tree base. The effects of microsite (Picea base versus Hylocomium carpet) on M. arizonicum performance were clearer in both life stages than the effects of elevation. Hylocomium microsites had consistently lower regeneration of apical fragments, but higher in situ effective quantum yield of adult transplants compared with Picea bases. Within the Picea base microsites, plots at intermediate elevations (1600-1800 m) received more light and were sub-optimal in terms of fragment regeneration, fragment sprout production, and in situ effective quantum yield of adult transplants. In the study area, the ability of M. arizonicum to extend its range into lower elevation sites was limited at intermediate elevations around tree bases that had deeper litter layers at intermediate elevation sites and in Hylocomium microsites by competitive pressure from this dominant forest floor moss.